Agr is a global regulator of a large set of staphylococcal genes, including many that are involved in pathogenesis. The agr locus encodes a two-component system composed of AgrC and A, and its autoinducing ligand (ALP), encoded by AgrD and processed by AgrB; its primary function is to activate the expression of a regulatory RNA that is the intracellular effector of target gene regulation. The AlP acts as a quorum sensing indicator and its accumulation stimulates the expression of several virulence factors. Natural variants of the AlP exist that cross-inhibit agr autoinduction in heterologous combinations, thus blocking pathogenesis. The goal of these studies is to examine the role of quorum sensing in the expression of virulence factors in the host during pathogenesis. To address this the specific aims are: 1. To determine the role of the Agr system in the pathogenesis of staphylococcal disease by following the expression of virulence genes during an experimental infection. 2. To characterize and identify regulatory loci that may be responsible for input into the temporal program of virulence gene expression. 3. To assess the effect of inhibitory AIPs on the Agr response, other regulatory loci and expression of virulence factors. Design and Methods: Gene fusions to a luciferase reporter will permit the monitoring of their expression as well as the relative biomass of the infecting population in a murine infection model, by means of a sensitive luciferase-detecting camera. The luciferase fusions can be assessed in various isogenic backgrounds to examine the role of ancillary factors on gene expression and pathogenesis. Inhibitory AIPs can be tested for delivery and modulating ability in vivo by assessing the effects on gene expression. A temporal model of virulence factor expression could provide insight into the pathogenic mechanisms of microbes and aid in the development of new antimicrobial therapies.